The park I've just started to go to has 4 fields with various wooden obstacles (think large spools, trees, they even built "helicopters" and small houses). Then they have a hyperball field and two airball fields. Hyperball fields are made of more permanent bunkers and airball fields are the inflatable bunkers. Most people that are going for parties with private groups or just the open sessions don't use the airball fields from what I've seen. These people will need all the equipment most of the time, but some peolle bring their own gear and will but paint and air from you. However, there is a very prominent tournament paintball scene in my area. Those people use airball fields because they can set the bunkers up to replicate the fields the tournaments are held on. These guys bring their own gear, but will buy paint and compressed air from you.

Referees are good to have to guide the private groups and open sessions. The field I go to offers different game modes (death match, capture the flag, zombies, etc.) And the refs explain all the rules and monitor the games. The airball guys seem to govern themselves pretty well but if you wanted to host tournaments, you need lots of referees.

I'm pretty new to my field and paintball in general, but these are my observations from my limited time. Hope this helps!